The use ofp-methylaminophenol sulphate (metol) at 100‡ for the reduction of phosphomolybdic acid in the determination of phosphate in sea-water has a number of advantages over stannous chloride. The molybdenum blue colour once formed, is stable for several hours, and the calibration curve is not dependent upon the batches of reagent used; Beer's law is obeyed up to concentrations of at least 1 mg PO4-P/l. The salt error is very small and the method has approximately 30% greater sensitivity in sea-water than has the stannous chloride method. Arsenic does not interfere at concentrations five times greater than its sea-water concentration. Iron and copper in moderate amounts do not interfere.